1. Field of the Invention
The present invention relates to a magnetic recording medium and, more particularly, to a magnetic recording medium having a coating layer in which a binder having improved properties is used.
2. Brief Description of the Prior Art
Thermoplastic polyurethane resins containing a terminal hydroxyl group or terminal hydroxyl groups, which may be employed particularly as binders for magnetic tapes or the like are used in the form of a solution in a solvent such as a ketone, e.g., acetone, methyl ethyl ketone, methyl isobutyl ketone or the like; an ester, e.g., ethyl acetate, butyl acetate or the like; an aromatic hydrocarbon, e.g., toluene, isophorone or the like; an alcohol, e.g., isopropyl alcohol or the like; or a mixture thereof. Such thermoplastic polyurethane resins, however, have the drawbacks that the solubility thereof in the solvent as hereinabove referred to becomes poorer in instances where the concentration of urethane groups in the polyurethane resin is rendered high by increasing the ratio of a low molecular weight diol in the resulting polyurethane resin in order to provide enhanced heat resistance and solvent resistance of the resulting thermoplastic polyurethane resin. Accordingly, in this case, the use of a solvent having a strong action to solubilize the polyurethane resin and a high polarity is required. Representatives of such solvents may be enumerated by methyl formamide, tetrahydrofuran or the like. Such solvent, however, may cause the disadvantage that it will corrode surface portions of a base film, a coated object or the like on which a product containing such solvent is brought into contact, whereby wrinkles or crinkles are partially caused or, in some cases, the portions may become dissolved. Therefore, there is a limitation on improvement by increasing the concentration of the urethane groups in the resulting thermoplastic polyurethane resin to be used.
As a process for improving, in particular, heat resistance and solvent resistance of a thermoplatic polyurethane resin, there is known a procedure in which a polyisocyanate such as a commercially available product "Coronate L" (manufactured and sold by Nippon Polyurethane Kogyo K. K.) is added to the thermoplastic polyurethane resin containing a terminal hydroxyl group or terminal hydroxyl groups and then the resulting mixture is subjected to chain prolongation or propagation and crosslinking reactions. In this process, both the hydroxyl groups present at the terminals of the thermoplastic polyurethane resin chain and the urethane bond or bonds present in the molecular chain thereof are allowed to react with the polyisocyanate. In particular, the reaction of the urethane bond with the isocyanate group of the polyisocyanate, which is called a allophanate reaction, that is, the reaction for forming the allophanate bond, requires high temperatures and it cannot provide a sufficient network structure in the resulting polyurethane resin where the resin is used to form a coating.
For binders for magnetic tapes and the like in which an inorganic filler or the like is dispersed or filled therein, workability at the time of coating, such as drying property, recoatability, curing velocity, liquid properties and the like is of great significance, in addition to various physical properties such as durability, adhesion to a base material and the like. On the top of those characteristic properties, thermoplastic polyurethane resins have been demanded to have desirable properties in respect of sedimentation, gloss on the surface of a cured product and the like. In particular, binders for magnetic tapes further require that the thermoplastic polyurethane resin to be used can provide favorable electromagnetic performance and properties resulting from the dispersibility of a pigment in the polyurethane resin. Conventional thermoplastic polyurethane resins, however, have the drawback that their action to disperse a pigment, an inorganic filler or the like is particularly poor so that thermoplastic polyurethane resins having particularly improved dispersion properties have been desired.
As a polyurethane resin possesses a resistance to wear to such an extent as other resins cannot provide, characteristic properties may be expected when such polyurethane resin is blended with the other resins. Conventional thermoplastic polyurethane resins, however, are not satisfactory in compatibility at all with the other resins.
As a method of improving the heat resistance and the solvent resistance of a thermoplastic polyurethane resin, while retaining favorable drying properties and flexibility originating from the polyurethane resin, there is known a method of providing a three-dimensional network structure by subjecting the resin to crosslinking between the polyurethane molecule chains. This method permits the formation of a three-dimensional network structure in the resulting polyurethane resin by adding a curing agent likely to be reactive with a functional group to the thermoplastic polyurethane resin which has at least two functional groups which can be reacted with the curing agent. Such thermoplastic polyurethane resins, however, do not provide the magnetic layer of a magnetic recording medium with sufficient dispersibility of magnetic or magnetizable particles or powders, wear resistance, curability and solvent resistance.
Since the pigment, the inorganic filler or the like to be contained in binders for magnetic tapes or the like has adsorption water and/or chemically bonded water on its surface even when dried to a sufficient extent, consequently it shows a hydrophilic property and compatibility with a compound having a hydroxyl group, a carboxyl group, a sulfone group, a tertiary amino group, a quanternary amino group or the like. As a result, the dispersibility of various binders or the like is improved by adsorption through the aid of those groups.